[codex] Add parser pretty-print round-trip tests

[jsiek]

Summary

• Add a parser pretty-print round-trip check to the existing --equiv harness mode.
• Round-trip representative accepted programs by parsing with RD/LALR, pretty-printing the AST, then re-parsing the generated source with both parsers and comparing canonical ASTs.
• Keep the fixture set curated because the current pretty-printer is not parse-preserving for the full corpus yet.
• Add mypy ignores for Markdown extension base classes in gh_pages/scripts/convert.py, which are imported as Any and block the required make tests gate.

Validation

• python3 test-deduce.py --equiv --serial
• python3 -m ruff check test-deduce.py gh_pages/scripts/convert.py
• python3 -m mypy .
• make tests

Draft PR for #473. This completes the round-trip parser test slice; the issue should remain open for broader grammar-fragment migration and any future expansion of the round-trip corpus.